secret base; hideout
Often used for children's play forts or hidden headquarters in adventure stories.
The children built a secret base in the woods.
He uses the basement as his hideout.
秘密基地 is a neutral, often playful term for a secret base (as used by children), whereas アジト carries a serious connotation of criminal or subversive intent.
Compound of 秘密 (secret) and 基地 (base).